This invention relates to gain control circuits and, in particular, to an improved balanced-type gain control circuit which is linearly controllable over a wide range and in which the DC level at the circuit output is maintained at a substantially constant value regardless of variations of circuit control current.
Balanced-type of gain control circuits are known, and especially balanced-type AGC circuits. These conventional AGC circuits consist of a differential amplifier formed of differentially connected transistors having common-connected emitters which are driven by a first current source. Usually, the input signal which is to be amplified is applied through an emitter-follower input transistor to one of the inputs of the differentially connected transistors. In addition, a second current source is coupled to the respective bases of the differentially connected transistors by forward-biased diodes.
Although this balanced-type of gain control circuit has been used in the prior art, the operation thereof is accompanied by notable disadvantages. For example, if the output DC level is to be maintained constant, the first-mentioned current source connected to the common-connected emitters must be a constant current source. Accordingly, the second-named current source connected to the transistor base electrodes will be varied to thereby permit control of the amplifier gain. However, if, during a gain control operation, the current level of the second-mentioned current source is reduced to a small level so as to effect a large amplifier gain, the output impedance of the emitter-follower transistor increases. The ratio of the current produced by the second-mentioned current source to the control voltage which is applied thereto cannot be maintained constant at such low current levels and, as a result, the amplifier current gain cannot be linearly controlled over a satisfactorily wide range.
On the other hand, if the amplifier current gain is to be linearly controlled, the second-mentioned current source must be a constant current source. Hence, in that event, the current produced by the first-mentioned current source (coupled to the common-connected emitters) is varied to control the amplifier gain accordingly. However, now the DC level at the amplifier output also will vary in response to this variation of the first-mentioned current source.